1. Field of the Invention
The present invention relates to the manufacture of semiconductor devices. More specifically, the present invention relates to plasma processing systems which deliver RF power to a plasma processing chamber.
2. Description of the Related Art
Semiconductor processing systems are used to process semiconductor wafers for fabrication of integrated circuits. In particular, plasma-based semiconductor processes are commonly used in etching, oxidation, chemical vapor deposition (CVD), etc. The plasma-based semiconductor processes are typically carried out by means of plasma processing systems and generally include a plasma processing chamber to provide a controlled setting.
FIG. 1 illustrates a schematic block diagram of an exemplary conventional plasma processing system 100 used for processing a semiconductor wafer 102. The plasma processing system 100 includes a plasma processing chamber 104, a shield box 106, a network match box 108, and an RF generator 110. The RF generator 110 is coupled to the match network box 108 by a co-axial cable 112. The shield box 106 is arranged to house or shield a co-axial cable 114, which couples the match network box 108 to the plasma processing chamber 104. A helium box 124, which is mounted on top of shield box 106, is used for supplying helium to the plasma processing chamber 104.
The plasma processing chamber 104 includes a shower head 116 and an electrostatic chuck 118. The shower head 116 is adapted to release a source gas into the chamber 104 for generating plasma over the wafer 102. The ESC 118 includes one or more dielectric layers 120 disposed over an electrode 122. The electrostatic chuck 118 functions to hold the wafer 102 in place for processing. The helium from the helium box 124 is provided through a port 140 to control the temperature of the wafer 102. The plasma processing system 100 also includes an ESC power supply (not shown) for supplying power to the ESC.
Electrostatic chucks are well known in the art and are amply described, for example, in commonly owned U.S. Pat. No. 5,789,904 by Francois Guyot and entitled "High Power Electrostatic Chuck Contact," U.S. patent application Ser. No. 08/624,988 by Jones et al. and entitled "Dynamic Feedback Electrostatic Wafer Chuck," U.S. patent application Ser. No. 08/550,510 by Castro et al., and U.S. Pat. No. 5,793,192 by Kubly et al. and entitled "Methods and Apparatus for Clamping and Declamping a Semiconductor Wafer in a Wafer Processing System." The disclosures of these references are incorporated herein by reference.
For wafer processing, the RF generator 110 provides RF power to the plasma processing chamber 104. Specifically, the RF generator 110 generates RF power, which is transmitted to the network match box 108 over the co-axial cable 112. The network match box 108 houses a matching network circuit 126, which produces an impedance match between the plasma processing chamber 104 and the RF generator 110 during wafer processing. The network match box 108 transmits RF power over the co-axial cable 114 to the plasma processing chamber 104. The matching network circuit is provided between the RF generator 106 and the plasma processing chamber 104 to minimize reflection of RF power from the plasma processing chamber 104. It typically includes two or more variable impedance elements (e.g., capacitors, inductors). RF match network circuits are well known in the art and are described, for example, in U.S. patent application No. 5,187,454 by Collins et al. and U.S. patent application Ser. No. 09/218,542 by Arthur M. Howald and filed on Dec. 22 1998. The disclosures of these references are incorporated herein by reference.
In high and medium density plasma etching, semiconductor manufacturers have been using electrical plasma parameters such as the DC bias voltage and the bias peak-to peak voltage to monitor the plasma processing in real time. For example, a DC voltage is typically developed over the wafer 102 and a peak-to-peak voltage can be measured from the electrode 122 during the operation of the plasma processing system 100. These electrical parameters are often used to diagnose, and if necessary, to interrupt the plasma process to achieve desired plasma processing. These electrical parameters are typically highly sensitive not only to the plasma density and plasma distribution inside the plasma chamber 104, but also to the spatial distribution of the RF return currents outside the plasma, i.e., through the walls of the plasma chamber 104, the RF delivery system, and the chassis of the RF matching network.
For example, the plasma processing system 100 of FIG. 1 delivers forward RF power to the electrode 122 of the ESC 118 via co-axial cable 114 as indicated by arrow 128. The RF power energizes the electrode 122, which attracts plasma ions toward the wafer 102 for plasma processing. The walls 130 of the plasma processing chamber 104 provide "return" paths for RF currents to return to the match network box 108 and eventually to the RF generator 110, thereby forming a closed circuit. Conventional wafer processing systems typically provide as much metal-to-metal surface contact to maximize RF current return paths from the plasma processing chamber 104 to the match network box 108. For instance, the plasma chamber 104, shield box 106, and the match network box 108 are typically formed of a metal (e.g., aluminum) for conducting electricity. Hence, RF currents travel from the walls 130 of the plasma chamber 104 as indicated by arrows 132 over any metal-to-metal contact paths leading to the match network box 108. Providing as much metal-to-metal contacts for return paths is in accordance with industry standard rule of thumb. For example, designers of plasma processing systems have typically tried to achieve the lowest impedance for RF return current. The lowest impedance is usually achieved by providing as much metal-to-metal contact in the plasma processing systems.
One of the main RF current return paths originates from the walls 130 to the match network box 108 along the coaxial cable 114 as indicated by arrows 134. In this case, the RF return currents travel along the surface of an outer conductor of the coaxial cable 114. In addition, RF currents also travel along other return paths over metal-to metal surfaces of the plasma processing chamber 104, the shield box, and the match network box 108. In this manner, the plasma processing system 100 is designed to provide as many return paths for RF currents as possible to capture as much stray currents as possible to ensure their return to the match network 126.
For high wafer yield, it is desirable to maintain consistent and uniform RF return currents in the plasma processing system 100. Unfortunately, however, providing such metal-to-metal contacts for maximum RF return paths degrades wafer processing over time. For example, the match network box 108 is securely attached to the shield box 106 using a plurality of bolts, screws, etc. through a metal plate (e.g., aluminum plate) disposed between the boxes 108 and 106. During the life of the plasma processing system 100, the match network box 108 is often removed from the shield box 106 for routine maintenance or modification. After the maintenance, the match network box is securely re-attached to the shield box by means of an aluminum plate and metal bolts, screws, etc.
The re-attachment of the match network box 108, however, generally does not precisely duplicate the metal-to-metal contacts existing prior to the removal. For example, the bolts or screws may not be screwed on exactly as before the removal. Thus, the changed metal-to-metal contact characteristics may change the RF current return path characteristics, which in turn lead to changes in the overall magnitude of RF return currents and in the electrical characteristics of the wafer processing. In addition, the use of aluminum for the plasma chamber walls 103 and the boxes 106 and 108 often lead to oxidation on the metal surface over time. That is, oxide layers may form on the metal surface, thereby changing the RF current return path characteristics.
In view of the foregoing, what is needed is an apparatus and method for providing consistent and uniform RF return currents in a plasma processing system to enhance the precision and uniformity of wafer processing results.